Demonstration of the formation of synaptic connections in vitro between fetal mouse dorsal-root ganglion (DRG) cells and specific types of target neurons in spinal cord and medulla explants provides a powerful new model system which offers insights into some of the cellular mechanisms regulating formation of specific neuronal connections in the CNS. Flexible experimental manipulation of the geometry and the physico-chemical environments of these neurons interacting in organized mammalian CNS cultures permits a more direct approach to problems in development of CNS networks which have heretofore been carried out primarily in situ during embryogenesis or regeneration of the CNS. Emphasis will be placed on mechanisms regulating growth and organization of fetal rodent sensory DRG cells in relation to specific types of CNS target neurons in dorsal-horn regions of spinal cord explants and dorsal-column nuclei of medulla explants. The role of NGF will be analyzed. Evidence of preferential growth of DRG neurites along chemical gradients will be sought. Systematic electrophysiologic and cytologic studies will be carried out during DRG-innervation of specific sensory-evoked networks in the target regions of cord and medulla explants. The functional neuronal components in these regionally specialized circuits will be analyzed, and attempts will be made to determine the extent to which the neurons may show positional, as well as phenotypic, specificity properties under these isolated conditions in vitro. Extension to retinotectal cultures is also in progress. Chronic effects of DRG-deafferentation of spinal cord and medulla explants on sensory target neurons will be studied. The capacity for re-formation of specific sensory-evoked synaptic networks in mature deafferented CNS explants will be determined by presentation of embryonic as well as regenerating dorsal root fibers from separate DRG ganglia.